1. Field of the Invention
The present invention relates to an image sensor, an electronic apparatus, and a driving method of an electronic apparatus, and more particularly, to an image sensor, an electronic apparatus, and a driving method of an electronic apparatus capable of obtaining a high-quality image with a simple configuration.
2. Description of the Related Art
A CMOS (complementary metal oxide semiconductor) image sensor and the like amplify signal charges accumulated in a photodiode, which is a photoelectric conversion element, with an active element and read the amplified signal charges as image information.
In an X-Y address type solid-state imaging device, such as a CMOS image sensor, for example, a plurality of pixel transistors are arrayed in a matrix. In addition, accumulation of signal charges corresponding to incident light is started for every line (row) or every pixel and signals having a current or a voltage based on the accumulated signal charges are read sequentially from each pixel by address designation. Thus, the X-Y address type solid-state imaging device is characterized in that the order of reading signals of pixels can be relatively freely set since a signal can be taken out from a pixel at an arbitrary position by address designation, unlike a CCD (charge coupled device) image sensor that sequentially reads signal charges obtained in pixels by selecting a pixel with a shift register.
For example, in a technique of imaging a still image represented by a digital still camera, an “all pixel read mode” in which a still image is obtained by independently reading pixel information on all pixels using a CMOS type solid-state imaging device, which has a plurality of pixels, as an imaging device is well known. In addition to the mode, an operation of, for example, a “thinning-out read mode” in which reading is performed while skipping some rows and columns can also be performed.
Moreover, in a CMOS image sensor, a shutter operation (electronic shutter) is performed in order to increase the precision of signal charges obtained in pixels. In response to the shutter operation, electric charges accumulated in pixels are once swept out. For example, in the case of reading a pixel on the basis of the “thinning-out read mode” mentioned above, a more complicated shutter operation is needed as a measure against leakage of electric charges from adjacent pixels and the like.
For example, in the thinning-out read mode, a so-called anti-blooming shutter for an operation of sweeping out electric charges accumulated in pixels on a row (thinned-out row) where reading of a signal of a pixel is not performed is required in addition to a shutter operation for specifying an exposure time. If the anti-blooming shutter is not performed, a sweeping operation on electric charges of pixels on a thinned-out row does not occur. Accordingly, electric charges overflow from a photodiode and the electric charges leak to adjacent pixels. As a result, a problem, such as color mixing, occurs.
Furthermore, a control for optimization of the exposure amount or the like is also performed by changing an exposure time based on an electronic shutter in the unit of a frame according to the brightness during a camera operation. In this case, the number of electronic shutters at the same timing may increase when the exposure amount has changed.
Thus, in the CMOS image sensor, selection of complicated read rows or shutter rows is needed. In order to actually give a large degree of freedom to a read order, a control mechanism that performs address designation according to each mode is needed. Accordingly, as a pixel row or column selection circuit, not a shift register but a decoder is used in many cases. The reason is that in the case of a shift register, rows or columns need to be sequentially selected to some extent, and accordingly, arbitrary rows or columns cannot be freely selected.
In the case of using a decoder, a vertical selection decoder that selects a row of pixels performs selection of a read row and selection of a shutter row in a 1 H period (one horizontal scanning period), such that read of electric charges of each pixel and a sweeping operation on electric charges are performed.
For example, in a ½ thinning-out read mode, when a change in exposure time caused by the above-described anti-blooming shutter and electronic shutter is taken into consideration, it is necessary to generate electronic shutter rows simultaneously at maximum four places at timing (1 H period) at which one read row is selected in order to obtain a high-quality image without poor exposure. Accordingly, since four decoders are needed to select four electronic shutter rows and one decoder is needed to select one read row, a total of five decoders are needed. As a result, the size of a circuit increases.
For this reason, it has been proposed to perform time-division multiplexing on addresses selected by a decoder such that a plurality of electronic shutter rows or read rows are selected with one decoder (for example, refer to JP-A-2004-166269).